Simian hemorrhagic fever (SHF) virus is an unclassified togavirus which resembles most closely the flaviviruses in its mode of replication. Four strains of virus have been identified. Two of the strains produce persistent infections in patas monkeys and the others, acute infections. All strains produce fatal infections of monkeys of the genus Macaca. By in vitro translation studies using rabbit reticulocyte lysates, we have obtained evidence that only non-virion polypeptides are coded from SHF virus genomic RNA. Four polypeptides with molecular weights of 40K, 25K, l5K and l2K daltons were synthesized from genomic RNA. Immunoprecipitation studies indicate that these polypeptides were not contained in mature virions. Polypeptides with these molecular weights were found in infected cells early in infection (up to three and one-half hours after infection). About five hours after infection, polypeptides contained inmature virions were found in infected cells. The factor(s) which control the regulation of synthesis of viron and non-virion polypeptides are currently unknown.